PuGa合金中H吸附行为的第一性原理研究北大核心CSCD

在氢气氛下,PuGa合金会发生氢化腐蚀反应,这会影响材料结构,导致PuGa合金相关性能下降。由于PuGa材料的高辐射性、相关实验技术及条件的制约,难以通过大规模的实验手段对此进行研究。因此,针对H在PuGa合金(111)面及体相内的吸附行为机制进行了第一性原理模拟研究,分析H在PuGa合金中的电子结构及吸附作用过程。研究发现H原子在PuGa体相中倾向吸附于包含Ga原子的四面体间隙,其吸附能为-0.75 eV,而在(111)面上倾向吸附于Pu原子周围,其最大吸附能绝对值为0_(6)2 eV。H在Ga原子邻近吸附位具有较低的扩散能垒,尤其是在表面上其反应能垒小于0.3 eV。本工作从原子尺度理解揭示H在材料表面的吸附行为机理,为降低H对材料结构及性能的影响提供理论指导。     

H原子在γ-U(100)表面吸附与扩散的密度泛函研究

采用密度泛函理论方法研究了H原子在γ-U(100)表面的吸附和扩散。结果表明：H原子在γ-U(100)表面的最佳吸附位依次为穴位、桥位和顶位,吸附能分别为2.696、2.597和2.017 eV;H原子在γ-U中的最佳间隙位为四面体间隙,其次是八面体间隙,吸附能分别为1.534 eV和0.991 eV;H原子在铀表面以及近表面层的不同吸附位之间的扩散需克服不同的势垒,相比于间隙位,H原子更倾向于在γ-U(100)表面吸附聚集。     

UO2中铌行为模拟研究

通过拟合Nb2O5的晶体结构,建立了铌的经验势.模拟计算孤立铌杂质表明:最近邻Nb-O间距为2.13A,均匀向铌靠近.近邻Nb-U间距为3.84(A),次近邻Nb-U间距为5.48(A),小于正常晶格的U-O间距以及U-U间距.铌附近间隙原子形成能全部增加.铌对周围氧原子施加了额外的束缚,氧间隙形成能随着Nb-O距离的增加而减小,但都大于完整的UO2的氧问隙形成能.引入少量铌元素后其影响范围扩大很多,采用静态过渡态理论计算的铌的空位机制扩散激活能为6.76eV,UO2中铌的扩散几乎不可能.多个铌杂质计算表明:铌离子倾向于团聚,形成替位缺陷簇.根据以上模拟计算,提出如下机制:铌以置换固溶体形式存在于UO2中,为了保持电中性,含铌的UO2中存在相应的氧间隙原子,氧化层总体上保持CaF2结构.铌形成二聚体或者四聚体,联结成网络,使氧的扩散势垒增大,降低了氧化速率.     

采用磁过滤阴极弧沉积技术制备含Ti类金刚石薄膜的研究

采用磁过滤阴极弧沉积技术以金属Ti为阴极在真空室中加入C2H2气体来制备含Ti类金刚石薄膜，研究不同的过滤弯管电流和在有过滤磁场存在条件下不同的C2H2气压对含Ti类金刚石薄膜的成分和组织结构的影响规律。实验结果表明：薄膜是由TiC和类金刚石两种结构组成的复合薄膜。C2H2气压和过滤弯管电流都对含Ti类金刚石薄膜的成分和组织结构有显著的影响。增加C2H2气压和过滤弯管电流都使薄膜中雨元素的原子百分含量逐渐下降，C元素的原子百分含量逐渐增加，TiC峰强度逐渐降低，半高宽逐渐增加。     

放射性核素U在针铁矿(010)表面吸附的第一性原理研究

采用基于密度泛函理论的第一性原理研究了有水与无水环境下放射性核素U在针铁矿(010)表面的吸附。结果表明,由于水分子与(010)表面的初始构型不同,其吸附能为-0.56~-0.85eV。同时发现在缺水的情况下,U原子可吸附在针铁矿(010)表面的替代位(T)、八面体间隙位(O)及两个O原子的桥位(B)上。其吸附能为负,大小按吸附T、B、O的顺序依次增大。水对U原子吸附能的影响较明显。当U原子占据替代位和八面体间隙位时,在其附近的水分子可增强U原子在针铁矿(010)表面的吸附,可使U原子在针铁矿(010)表面的吸附能降低1~2eV;当U原子在桥位时,水分子既可增强也可减弱U原子在针铁矿(010)表面的吸附能力,这依赖于U原子与水分子作用后其价态的变化。所有这些吸附均为放热反应。计算结果显示,由于水分子与U原子的吸附引起的表面晶格畸变较小,所有涉及的表面键长变化均不大于0.055nm。所有涉及到的反应过程中均有H、U、Fe原子上的部分电子转移到O原子上。     

H原子在Zr(0001)表面吸附的第一性原理研究

结合蒙特卡洛（MC）模拟和第一性原理密度泛函理论（DFT）方法，从Zr-H体系微观结构、吸附概率、吸附能、Mulliken电荷布居数以及电子态密度等方面对H原子在α-Zr(0001)表面的吸附位点和吸附机理等进行计算分析。结果表明:H原子在Zr(0001)表面首先产生物理吸附，然后由物理吸附转变为化学吸附，吸附过程中电荷不断由 Zr(0001)表面原子向H原子转移，最后趋于稳定。另外，稳定吸附后的H原子直接与 Zr(0001)表面最表层原子生成化学键，且主要由H(s)、Zr(s)和Zr(d)轨道的电子态做贡献。综合分析得到H原子在Zr(0001)表面的吸附位点优先级顺序为密排六方间隙位（hcp位）＞面心立方间隙位（fcc位）＞桥位（bridge位），顶位（top位）不会产生吸附。      

TiC/Ti(C,N)/TiN复合涂层在滑移区的高温微动磨损特性

采用化学气相沉积（CVD）技术，在1Cr13不锈钢基体上制备TiC／Ti（C，N）／TiN复合涂层，研究了该涂层从室温（25℃）到400℃的微动磨损特性，并与无涂层的1Cr13不锈钢基材比较。结果表明：在滑移区．温度对1Cr13不锈钢微动磨损影响较显著；随着温度上升1Cr13不锈钢摩擦系数减少，磨损体积下降；而温度对TiC／Ti（C，N）／TiN复合涂层磨损体积影响不大．且磨损量均很小．TiC/Ti（C，N）／TiN涂层表现出比1Cr13不锈钢优异的抗微动磨损性能。但当该涂层被磨去后，产生的硬质磨屑形成磨粒磨损，反而对基体造成更大的损伤。     

Ti/Mo膜的X射线光电子能谱分析

利用X射线光电子谱仪（XPS）分析和Ar^ 刻蚀相结合的方法，分析了Ti膜表面的化学元素及相应原子的电子结合能。分析结果表明：Ti膜及膜材料样品表面有大量的C、O元素；膜表面存在从衬底扩散至Ti膜的Mo元素。对样品刻蚀后Ti2p的XPS谱进行拟合表明：Ti膜表面的Ti由TiO2（约100％）和单质Ti组成，随刻蚀时间的增加，部分TiO2还原至低价Ti；薄的薄膜表面中的Mo由单质Mo和MoO3组成，而厚的薄膜以单质Mo为主；表面C由石墨态和结合能为288．2－288．9eV的碳化物组成。     

Si原子在CeO2(111)表面吸附与迁移的第一性原理研究

为探究Si原子在CeO₂(111)表面吸附的微观行为，采用第一性原理的方法研究了Si原子在CeO₂(111)表面的吸附作用、电子结构和迁移过程，计算了Si原子在CeO₂(111)表面的吸附能，最稳定及次稳定吸附位置的电子态密度与电荷密度分布、迁移激活能。计算结果表明：Si原子最易吸附于基底表层的O原子上，其中O桥位（O_bri）吸附作用最强，O顶位（O_t）和O三度位（O_h）吸附强度次之。Si原子仅对其最邻近的表层O原子结构影响较大，这与Si原子及其最邻近的O原子间电荷密度重叠程度增强的结果一致。Si原子最易围绕着O_t位从O_bri位向O_h位迁移，迁移所需激活能为0.849 eV。     

水与三甲基硅烷醇羟基的氢同位素交换反应的理论研究

以(CH_3)_3SiOH羟基模拟Li_4SiO_4陶瓷表面羟基,研究了H_2O与(CH_3)_3SiOH羟基H的氢交换反应机理.采用HF, MP2方法,在3-21G和6-311G++H~(**)水平上优化了(CH_3)_3SiOH, H_2O, (CH_3)_3SiOH-H_2O复合物及氢交换反应过渡态的结构.计算了生成(CH_3)_3SiOH-H_2O复合物的反应热,探讨了氢交换反应的路径.结果表明,可以形成2种形式的(CH_3)_3SiOH-H_2O复合物,一种是H_2O的O原子与(CH_3)_3SiOH羟基的H原子作用形成的复合物,另一种是H_2O的H原子与(CH_3)_3SiOH羟基的O原子作用形成的复合物.MP2/6-311G++~(**)水平上,对基组重叠能(BSSE)进行校正后,上述2种复合物的反应热分别为20.046 5 kJ/mol和21.630 7 kJ/mol.有利的氢交换反应路径为:H_2O的H原子与(CH_3)_3SiOH羟基的O原子作用形成的复合物,然后H2O提供1个H原子、1个O原子,(CH_3)_3SiOH提供1个O原子、1个Si原子形成由O, H, O, Si 4个原子构成的四元环过渡态,最后H_2O的O原子与(CH_3)_3SiOH 的Si原子成键形成新的(CH_3)_3SiOH,而(CH_3)_3SiOH的Si-O键断裂,由(CH_3)_3SiOH的羟基和H_2O的1个H原子形成新的H_2O分子,MP2/6-311G++~(**)水平上,BSSE校正后,此路径的反应活化能为186.898 4 kJ/mol.     

Stability and migration property of helium and self defects in vanadium and V-4Cr-4Ti alloy by first-principles

The stability and migration behavior of helium and self defects in vanadium and V-4Cr-4Ti alloy are studied by first-principles calculations. The tetrahedral site is found as the most stable configuration for interstitial He, followed by the octahedral and substitutional sites. Among the self defects, the monovacancy has lower formation energy (1.71 eV for V and 2.14 eV for V-4Cr-4Ti alloy) than the self interstitial ones. The migration energies for He hopping between the tetrahedral sites are 0.06 and 0.09 eV for vanadium and V-4Cr-4Ti alloy, respectively. Our calculations reveal strong repulsion between two interstitial He atoms and strong attraction between He and vacancy, suggesting that vacancy acts as a trapping site for He impurity and a seed for further bubble formation.     

Atomistic properties of helium in hcp titanium: A first-principles study

First-principles calculations based on density functional theory have been performed to investigate the behaviors of He in hcp-type Ti. The most favorable interstitial site for He is not an ordinary octahedral or tetrahedral site, but a novel interstitial site (called FC) with a formation energy as low as 2.67 eV, locating the center of the face shared by two adjacent octahedrons. The origin was further analyzed by composition of formation energy of interstitial He defects and charge density of defect-free hcp Ti. It has also been found that an interstitial He atom can easily migrate along 〈0 0 1〉 direction with an activation energy of 0.34 eV and be trapped by another interstitial He atom with a high binding energy of 0.66 eV. In addition, the small He clusters with/without Ti vacancy have been compared in details and the formation energies of He_nV clusters with a pre-existing Ti vacancy are even higher than those of He_n clusters until n ? 3.     

Chemical states and thermal stability of hydrogen-implanted Ti and V studied by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was applied to the chemical state studies of Ti and V metals bombarded with 8 keV hydrogen ions. The binding energies of the lines for the ion-implanted Ti and V shift from those for the metallic states by 0.3 eV, are consistent with the core-line shifts for the thermally synthesized hydrides such as TiH_1.97 and VH_0.55 The metal 3d-H1s bonding level for the hydrogen implanted Ti appears at ~ 3.5 eV below the Fermi level, which is lower by ~ 2.5 eV than the molecular-orbital energy previously calculated for TiH₂. In the case of the ion-implanted V, however, it appears at ~ 5.0 eV, which is almost equal to the molecular-orbital energy for VH₂. The photopeaks corresponding to the Ti-H and V-H bonds for the ion-implanted samples grew up on raising the annealing temperature up to 550°C and 150°C, respectively. The phenomena are interpreted by means of the thermal diffusion of the implanted hydrogen from bulk to surface.     

Ag diffusion in cubic silicon carbide

The diffusion of Ag impurities in bulk 3C-SiC is studied using ab initio methods based on density functional theory. This work is motivated by the desire to reduce transport of radioactive Ag isotopes through the SiC boundary layer in the Tristructural-Isotropic (TRISO) fuel pellet, which is a significant concern for the Very High Temperature Reactor (VHTR) nuclear reactor concept. The structure and stability of charged Ag and Ag-vacancy clusters in SiC are calculated. Relevant intrinsic SiC defect energies are also determined. The most stable state for the Ag impurity in SiC is found to be a Ag atom substituting on the Si sub-lattice and bound to a C vacancy. Bulk diffusion coefficients are estimated for different impurity states and values are all found to have very high activation energy. The impurity state with the lowest activation energy for diffusion is found to be the Ag interstitial, with an activation energy of approximately 7.9 eV. The high activation energies for Ag diffusion in bulk 3C-SiC cause Ag transport to be very slow in the bulk and suggests that observed Ag transport in this material is due to an alternative mechanism (e.g., grain boundary diffusion).     

Calculations of the trapping and migration of vacancies and nickel self-interstitials in the presence of rare gases and dislocations

Recent models of swelling, void growth, and solute segregation under irradiation all require knowledge of the trapping and migration of vacancies and self-interstitials in the presence of lattice defects. The present calculations include trapping of both vacancies and nickel self-interstitials to substitutional and interstitial rare gas atoms. The results show a systematic dependence on rare gas atom size. It is found for example, that a vacancy is bound to a small fixed rare gas interstitial (He) by ～0.5 eV and to a large fixed interstitial (Xe) by ≥3 eV. In addition, a fixed substitutional rare gas or rare gas interstitial is found to be a strong trap for a self-interstitial. It is found that a single vacancy can significantly affect the migration energy of another vacancy. For example, a 0.4 eV decrease in migration energy is found at a distance of three half-lattice constants. However, this interaction is of limited range; at distances greater than five half-lattice constants vacancy migration is unaffected. The migration of vacancies near the core of a partial dislocation was also investigated. This partial is found to provide a 1 eV (compared to 1.4 eV in the bulk) path for the pipe diffusion of vacancies. In addition, the activation energy for vacancy migration along the slip plane is reduced by as much as 0.2 eV.     

Modelling of deposition processes on the TiO2 rutile (1 1 0) surface

Deposition of Ti_xO_y clusters onto the rutile TiO₂ (1 1 0) surface has been modelled using empirical potential based molecular dynamics. Deposition energies in the range 10-40 eV have been considered so as to model typical deposition energies of magnetron sputtering. Defects formed as a function of both the deposition energy and deposition species have been studied.The results show that in the majority of cases Ti interstitial atoms are formed, irrespective of whether Ti was contained within the deposited cluster. Furthermore that the majority of these interstitials are formed by displacing a surface Ti atom into the interstitial site. O surface atoms are also relatively common, with Ti and TiO₂ surface units often occurring when the deposited cluster contains Ti but becoming less frequent as the deposition energy is increased. Structures that would give rise to the growth of further layers of rutile are not observed and in the majority of the simulations the energy barriers for diffusion of the end-products is high.     

First-principles energy and vibration spectrum simulations of Cr/V interacting with H in W-based alloy in a fusion reactor

We perform first-principles total energy and vibration spectrum calculations to study the effect of Cr/V on the H formation and diffusion at temperature range 300–2100 K in dilute W–Cr/W–V alloy. Temperature and H chemical potential are two important factors to affect the H formation energy and migrating energy. The H formation energy referring to the static/temperature-dependent H chemical potential decreases/increases with the temperature. At each given temperature, the presence of Cr in W reduces the H formation energy, while the existence of V in W has little effect on the H formation energy. The diffusion energy barrier and pre-exponential factor strongly depend on the temperature and increase with the temperature from 300 to 2100 K. Both Cr and V additions in W has a large consequences on H migrating energy. The energy barriers at any given temperature can be reduced by ～0.05 and 0.10 eV in W–Cr and W–V alloys, respectively. The current study reveals that vibration free energy plays a decisive role in the formation energy and migrating energy of H with the temperature, while the thermal expansion energy has little influence on the H formation energy and migrating energy with the temperature.     

First-principles investigation on dissolution and diffusion of oxygen in tungsten

Using a first-principles method, we have investigated dissolution and diffusion properties of oxygen (O) in tungsten (W). Single O atom prefers to occupy the tetrahedral interstitial site (TIS). Two interstitial O atoms are attractive and tend to be paired up at two neighboring TIS with a distance of 0.228 nm and a large binding energy of 1.60 eV, which indicates a strong tendency of O clustering in W. O is preferred to diffuse between the most nearest neighboring TIS with a diffusion barrier of 0.17 eV. By the estimation of pre-exponential factor according to an empirical theory, the diffusion coefficient as a function of temperature has been determined, which is 1.50 × 10⁻⁹ m²/s at a typical temperature of 500 K. The results provide a good reference to understand the behavior of O in intrinsic W.